In vivo Nuclear magnetic resonance (NMR) is rapidly evolving as a critical tool as it offers real-time metabolic information, which is crucial for delineating complex toxic response pathways in living systems. Organisms such as D. magna (water fleas) and H.azteca (freshwater shrimps) are commonly 13C enriched to increase signal in NMR experiments. A key goal of in vivo NMR is to monitor how molecules (nutrients, contaminants, or drugs) are metabolized. Conventionally, these studies would normally involve using a 13C enriched probe molecule and feeding this to an organism at natural abundance, in turn allowing the fate of the probe molecule to be selectively analyzed. The setback of such an approach is that there is a limited range of 13C enriched probe molecules, and if available, are extremely cost prohibitive. Uniquely, when utilizing 13C organisms a reverse strategy of isotopic filtering becomes possible. The concept described here uses 1H detection in combination with a 13C filter on living organisms....